Effects of hydrogen peroxide on intergranular stress corrosion cracking ofstainless steel in high temperature water, (IV) Effects of oxide film on electrochemical corrosion potential

In order to determine the effects of hydrogen peroxide. on electrochemical corrosion potential (ECP) of type 304 stainless steel (SUS304), ECPs were m easured using a high temperature, high pressure water loop with polytetrafl uoroethylene (PTFE) inner liner at controlled hydrogen peroxide concentrati on. It is observed that the ECP of SUS304 exposed to hydrogen peroxide is h igher than that when exposed to oxygen at the same oxidant concentration. T he ECP shows a hysteresis pattern for its concentration dependency. Those r esults were attributed mainly from the chemical form of oxide film on stain less steel specimens. The oxide film was affected by the corrosive circumst ances. Hematite (alpha -Fe2O3) was observed for the specimens exposed to hy drogen peroxide, while Fe3O4 was a main oxide when exposed to oxygen. The d ifference of the anodic polarization curves between O-2 and H2O2 environmen ts was caused by the difference of the stability between alpha -Fe2O3 and F e3O4. Since the alpha -Fe2O3 is reduced to the Fe2+ when hydrogen is added to water, the ECP decreases with decreasing oxidant concentration without s howing the hysteresis that keep the ECP higher value.